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Flutter的原理及美团的实践(中)

日期:2019-12-11点击:406

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外卖全品类页面实践

在调研了Flutter的各项特性和实现原理之后,外卖计划灰度上线Flutter版的全品类页面。对于将Flutter页面作为App的一部分这种集成模式,官方并没有提供完善的支持,所以我们首先需要了解Flutter是如何编译、打包并且运行起来的。

Flutter App构建过程

最简单的Flutter工程至少包含两个文件:

运行Flutter程序时需要对应平台的宿主工程,在Android上Flutter通过自动创建一个Gradle项目来生成宿主,在项目目录下执行flutter create .,Flutter会创建ios和android两个目录,分别构建对应平台的宿主项目,Android目录内容如下:

此Gradle项目中只有一个app module,构建产物即是宿主APK。Flutter在本地运行时默认采用Debug模式,在项目目录执行flutter run即可安装到设备中并自动运行,Debug模式下Flutter使用JIT方式来执行Dart代码,所有的Dart代码都会打包到APK文件中assets目录下,由libflutter.so中提供的DartVM读取并执行:

kernel_blob.bin是Flutter引擎的底层接口和Dart语言基本功能部分代码:

third_party/dart/runtime/bin/*.dart third_party/dart/runtime/lib/*.dart third_party/dart/sdk/lib/_http/*.dart third_party/dart/sdk/lib/async/*.dart third_party/dart/sdk/lib/collection/*.dart third_party/dart/sdk/lib/convert/*.dart third_party/dart/sdk/lib/core/*.dart third_party/dart/sdk/lib/developer/*.dart third_party/dart/sdk/lib/html/*.dart third_party/dart/sdk/lib/internal/*.dart third_party/dart/sdk/lib/io/*.dart third_party/dart/sdk/lib/isolate/*.dart third_party/dart/sdk/lib/math/*.dart third_party/dart/sdk/lib/mirrors/*.dart third_party/dart/sdk/lib/profiler/*.dart third_party/dart/sdk/lib/typed_data/*.dart third_party/dart/sdk/lib/vmservice/*.dart flutter/lib/ui/*.dart 

platform.dill则是实现了页面逻辑的代码,也包括Flutter Framework和其他由pub依赖的库代码:

flutter_tutorial_2/lib/main.dart flutter/packages/flutter/lib/src/widgets/*.dart flutter/packages/flutter/lib/src/services/*.dart flutter/packages/flutter/lib/src/semantics/*.dart flutter/packages/flutter/lib/src/scheduler/*.dart flutter/packages/flutter/lib/src/rendering/*.dart flutter/packages/flutter/lib/src/physics/*.dart flutter/packages/flutter/lib/src/painting/*.dart flutter/packages/flutter/lib/src/gestures/*.dart flutter/packages/flutter/lib/src/foundation/*.dart flutter/packages/flutter/lib/src/animation/*.dart .pub-cache/hosted/pub.flutter-io.cn/collection-1.14.6/lib/*.dart .pub-cache/hosted/pub.flutter-io.cn/meta-1.1.5/lib/*.dart .pub-cache/hosted/pub.flutter-io.cn/shared_preferences-0.4.2/*.dart 

kernel_blob.bin和platform.dill都是由flutter_tools中的bundle.dart中调用KernelCompiler生成。

在Release模式(flutter run --release)下,Flutter会使用Dart的AOT运行模式,编译时将Dart代码转换成ARM指令:

kernel_blob.bin和platform.dill都不在打包后的APK中,取代其功能的是(isolate/vm)_snapshot_(data/instr)四个文件。snapshot文件由Flutter SDK中的flutter/bin/cache/artifacts/engine/android-arm-release/darwin-x64/gen_snapshot命令生成,vm_snapshot_是Dart虚拟机运行所需要的数据和代码指令,isolate_snapshot_则是每个isolate运行所需要的数据和代码指令。

Flutter App运行机制

Flutter构建出的APK在运行时会将所有assets目录下的资源文件解压到App私有文件目录中的flutter目录下,主要包括处理字符编码的icudtl.dat,还有Debug模式的kernel_blob.bin、platform.dill和Release模式下的4个snapshot文件。默认情况下Flutter在Application#onCreate时调用FlutterMain#startInitialization来启动解压任务,然后在FlutterActivityDelegate#onCreate中调用FlutterMain#ensureInitializationComplete来等待解压任务结束。

Flutter在Debug模式下使用JIT执行方式,主要是为了支持广受欢迎的热刷新功能:

触发热刷新时Flutter会检测发生改变的Dart文件,将其同步到App私有缓存目录下,DartVM加载并且修改对应的类或者方法,重建控件树后立即可以在设备上看到效果。

在Release模式下Flutter会直接将snapshot文件映射到内存中执行其中的指令:

在Release模式下,FlutterActivityDelegate#onCreate中调用FlutterMain#ensureInitializationComplete方法中会将AndroidManifest中设置的snapshot(没有设置则使用上面提到的默认值)文件名等运行参数设置到对应的C++同名类对象中,构造FlutterNativeView实例时调用nativeAttach来初始化DartVM,运行编译好的Dart代码。

打包Android Library

了解Flutter项目的构建和运行机制后,我们就可以按照其需求打包成AAR然后集成到现有原生App中了。首先在andorid/app/build.gradle中修改:

简单修改后我们就可以使用Android Studio或者Gradle命令行工具将Flutter代码打包到aar中了。Flutter运行时所需要的资源都会包含在aar中,将其发布到maven服务器或者本地maven仓库后,就可以在原生App项目中引用。

但这只是集成的第一步,为了让Flutter页面无缝衔接到外卖App中,我们需要做的还有很多。

图片资源复用

Flutter默认将所有的图片资源文件打包到assets目录下,但是我们并不是用Flutter开发全新的页面,图片资源原来都会按照Android的规范放在各个drawable目录,即使是全新的页面也会有很多图片资源复用的场景,所以在assets目录下新增图片资源并不合适。

Flutter官方并没有提供直接调用drawable目录下的图片资源的途径,毕竟drawable这类文件的处理会涉及大量的Android平台相关的逻辑(屏幕密度、系统版本、语言等等),assets目录文件的读取操作也在引擎内部使用C++实现,在Dart层面实现读取drawable文件的功能比较困难。Flutter在处理assets目录中的文件时也支持添加多倍率的图片资源,并能够在使用时自动选择,但是Flutter要求每个图片必须提供1x图,然后才会识别到对应的其他倍率目录下的图片:

flutter: assets: - images/cat.png - images/2x/cat.png - images/3.5x/cat.png new Image.asset('images/cat.png'); 

这样配置后,才能正确地在不同分辨率的设备上使用对应密度的图片。但是为了减小APK包体积我们的位图资源一般只提供常用的2x分辨率,其他分辨率的设备会在运行时自动缩放到对应大小。针对这种特殊的情况,我们在不增加包体积的前提下,同样提供了和原生App一样的能力:

  1. 在调用Flutter页面之前将指定的图片资源按照设备屏幕密度缩放,并存储在App私有目录下。
  2. Flutter中使用时通过自定义的WMImage控件来加载,实际是通过转换成FileImage并自动设置scale为devicePixelRatio来加载。

这样就可以同时解决APK包大小和图片资源缺失1x图的问题。

Flutter和原生代码的通信

我们只用Flutter实现了一个页面,现有的大量逻辑都是用Java实现,在运行时会有许多场景必须使用原生应用中的逻辑和功能,例如网络请求,我们统一的网络库会在每个网络请求中添加许多通用参数,也会负责成功率等指标的监控,还有异常上报,我们需要在捕获到关键异常时将其堆栈和环境信息上报到服务器。这些功能不太可能立即使用Dart实现一套出来,所以我们需要使用Dart提供的Platform Channel功能来实现Dart→Java之间的互相调用。

以网络请求为例,我们在Dart中定义一个MethodChannel对象:

import 'dart:async'; import 'package:flutter/services.dart'; const MethodChannel _channel = const MethodChannel('com.sankuai.waimai/network'); Future<Map<String, dynamic>> post(String path, [Map<String, dynamic> form]) async { return _channel.invokeMethod("post", {'path': path, 'body': form}).then((result) { return new Map<String, dynamic>.from(result); }).catchError((_) => null); } 

然后在Java端实现相同名称的MethodChannel:

public class FlutterNetworkPlugin implements MethodChannel.MethodCallHandler { private static final String CHANNEL_NAME = "com.sankuai.waimai/network"; @Override public void onMethodCall(MethodCall methodCall, final MethodChannel.Result result) { switch (methodCall.method) { case "post": RetrofitManager.performRequest(post((String) methodCall.argument("path"), (Map) methodCall.argument("body")), new DefaultSubscriber<Map>() { @Override public void onError(Throwable e) { result.error(e.getClass().getCanonicalName(), e.getMessage(), null); } @Override public void onNext(Map stringBaseResponse) { result.success(stringBaseResponse); } }, tag); break; default: result.notImplemented(); break; } } } 

在Flutter页面中注册后,调用post方法就可以调用对应的Java实现:

loadData: (callback) async { Map<String, dynamic> data = await post("home/groups"); if (data == null) { callback(false); return; } _data = AllCategoryResponse.fromJson(data); if (_data == null || _data.code != 0) { callback(false); return; } callback(true); }), 

SO库兼容性

Flutter官方只提供了四种CPU架构的SO库:armeabi-v7a、arm64-v8a、x86和x86-64,其中x86系列只支持Debug模式,但是外卖使用的大量SDK都只提供了armeabi架构的库。

虽然我们可以通过修改引擎src根目录和third_party/dart目录下build/config/arm.gnithird_party/skia目录下的BUILD.gn等配置文件来编译出armeabi版本的Flutter引擎,但是实际上市面上绝大部分设备都已经支持armeabi-v7a,其提供的硬件加速浮点运算指令可以大大提高Flutter的运行速度,在灰度阶段我们可以主动屏蔽掉不支持armeabi-v7a的设备,直接使用armeabi-v7a版本的引擎。

做到这点我们首先需要修改Flutter提供的引擎,在Flutter安装目录下的bin/cache/artifacts/engine下有Flutter下载的所有平台的引擎:

我们只需要修改android-arm、android-arm-profile和android-arm-release下的flutter.jar,将其中的lib/armeabi-v7a/libflutter.so移动到lib/armeabi/libflutter.so即可:

cd $FLUTTER_ROOT/bin/cache/artifacts/engine for arch in android-arm android-arm-profile android-arm-release; do pushd $arch cp flutter.jar flutter-armeabi-v7a.jar # 备份 unzip flutter.jar lib/armeabi-v7a/libflutter.so mv lib/armeabi-v7a lib/armeabi zip -d flutter.jar lib/armeabi-v7a/libflutter.so zip flutter.jar lib/armeabi/libflutter.so popd done 

这样在打包后Flutter的SO库就会打到APK的lib/armeabi目录中。在运行时如果设备不支持armeabi-v7a可能会崩溃,所以我们需要主动识别并屏蔽掉这类设备,在Android上判断设备是否支持armeabi-v7a也很简单:

public static boolean isARMv7Compatible() { try { if (SDK_INT >= LOLLIPOP) { for (String abi : Build.SUPPORTED_32_BIT_ABIS) { if (abi.equals("armeabi-v7a")) { return true; } } } else { if (CPU_ABI.equals("armeabi-v7a") || CPU_ABI.equals("arm64-v8a")) { return true; } } } catch (Throwable e) { L.wtf(e); } return false; } 

灰度和自动降级策略

Horn是一个美团内部的跨平台配置下发SDK,使用Horn可以很方便地指定灰度开关:

在条件配置页面定义一系列条件,然后在参数配置页面添加新的字段flutter即可:

因为在客户端做了ABI兜底策略,所以这里定义的ABI规则并没有启用。

Flutter目前仍然处于Beta阶段,灰度过程中难免发生崩溃现象,观察到崩溃后再针对机型或者设备ID来做降级虽然可以尽量降低影响,但是我们可以做到更迅速。外卖的Crash采集SDK同时也支持JNI Crash的收集,我们专门为Flutter注册了崩溃监听器,一旦采集到Flutter相关的JNI Crash就立即停止该设备的Flutter功能,启动Flutter之前会先判断FLUTTER_NATIVE_CRASH_FLAG文件是否存在,如果存在则表示该设备发生过Flutter相关的崩溃,很有可能是不兼容导致的问题,当前版本周期内在该设备上就不再使用Flutter功能。

除了崩溃以外,Flutter页面中的Dart代码也可能发生异常,例如服务器下发数据格式错误导致解析失败等等,Dart也提供了全局的异常捕获功能:

import 'package:wm_app/plugins/wm_metrics.dart'; void main() { runZoned(() => runApp(WaimaiApp()), onError: (Object obj, StackTrace stack) { uploadException("$obj\n$stack"); }); } 

这样我们就可以实现全方位的异常监控和完善的降级策略,最大程度减少灰度时可能对用户带来的影响。

分析崩溃堆栈和异常数据

Flutter的引擎部分全部使用C/C++实现,为了减少包大小,所有的SO库在发布时都会去除符号表信息。和其他的JNI崩溃堆栈一样,我们上报的堆栈信息中只能看到内存地址偏移量等信息:

*** *** *** *** *** *** *** *** *** *** *** *** *** *** *** *** Build fingerprint: 'Rock/odin/odin:7.1.1/NMF26F/1527007828:user/dev-keys' Revision: '0' Author: collect by 'libunwind' ABI: 'arm64-v8a' pid: 28937, tid: 29314, name: 1.ui >>> com.sankuai.meituan.takeoutnew <<< signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 0x0 backtrace: r0 00000000 r1 ffffffff r2 c0e7cb2c r3 c15affcc r4 c15aff88 r5 c0e7cb2c r6 c15aff90 r7 bf567800 r8 c0e7cc58 r9 00000000 sl c15aff0c fp 00000001 ip 80000000 sp c0e7cb28 lr c11a03f9 pc c1254088 cpsr 200c0030 #00 pc 002d7088 /data/app/com.sankuai.meituan.takeoutnew-1/lib/arm/libflutter.so #01 pc 002d5a23 /data/app/com.sankuai.meituan.takeoutnew-1/lib/arm/libflutter.so #02 pc 002d95b5 /data/app/com.sankuai.meituan.takeoutnew-1/lib/arm/libflutter.so #03 pc 002d9f33 /data/app/com.sankuai.meituan.takeoutnew-1/lib/arm/libflutter.so #04 pc 00068e6d /data/app/com.sankuai.meituan.takeoutnew-1/lib/arm/libflutter.so #05 pc 00067da5 /data/app/com.sankuai.meituan.takeoutnew-1/lib/arm/libflutter.so #06 pc 00067d5f /data/app/com.sankuai.meituan.takeoutnew-1/lib/arm/libflutter.so #07 pc 003b1877 /data/app/com.sankuai.meituan.takeoutnew-1/lib/arm/libflutter.so #08 pc 003b1db5 /data/app/com.sankuai.meituan.takeoutnew-1/lib/arm/libflutter.so #09 pc 0000241c /data/data/com.sankuai.meituan.takeoutnew/app_flutter/vm_snapshot_instr 

单纯这些信息很难定位问题,所以我们需要使用NDK提供的ndk-stack来解析出具体的代码位置:

ndk-stack -sym PATH [-dump PATH] Symbolizes the stack trace from an Android native crash. -sym PATH sets the root directory for symbols -dump PATH sets the file containing the crash dump (default stdin) 

如果使用了定制过的引擎,必须使用engine/src/out/android-release下编译出的libflutter.so文件。一般情况下我们使用的是官方版本的引擎,可以在flutter_infra页面直接下载带有符号表的SO文件,根据打包时使用的Flutter工具版本下载对应的文件即可。比如0.4.4 beta版本:

$ flutter --version # version命令可以看到Engine对应的版本 06afdfe54e Flutter 0.4.4 • channel beta • https://github.com/flutter/flutter.git Framework • revision f9bb4289e9 (5 weeks ago) • 2018-05-11 21:44:54 -0700 Engine • revision 06afdfe54e Tools • Dart 2.0.0-dev.54.0.flutter-46ab040e58 $ cat flutter/bin/internal/engine.version # flutter安装目录下的engine.version文件也可以看到完整的版本信息 06afdfe54ebef9168a90ca00a6721c2d36e6aafa 06afdfe54ebef9168a90ca00a6721c2d36e6aafa 

拿到引擎版本号后在https://console.cloud.google.com/storage/browser/flutter_infra/flutter/06afdfe54ebef9168a90ca00a6721c2d36e6aafa/ 看到该版本对应的所有构建产物,下载android-arm-release、android-arm64-release和android-x86目录下的symbols.zip,并存放到对应目录:

执行ndk-stack即可看到实际发生崩溃的代码和具体行数信息:

ndk-stack -sym flutter-production-syms/06afdfe54ebef9168a90ca00a6721c2d36e6aafa/armeabi-v7a -dump flutter_jni_crash.txt ********** Crash dump: ********** Build fingerprint: 'Rock/odin/odin:7.1.1/NMF26F/1527007828:user/dev-keys' pid: 28937, tid: 29314, name: 1.ui >>> com.sankuai.meituan.takeoutnew <<< signal 11 (SIGSEGV), code 1 (SEGV_MAPERR), fault addr 0x0 Stack frame #00 pc 002d7088 /data/app/com.sankuai.meituan.takeoutnew-1/lib/arm/libflutter.so: Routine minikin::WordBreaker::setText(unsigned short const*, unsigned int) at /b/build/slave/Linux_Engine/build/src/out/android_release/../../flutter/third_party/txt/src/minikin/WordBreaker.cpp:55 Stack frame #01 pc 002d5a23 /data/app/com.sankuai.meituan.takeoutnew-1/lib/arm/libflutter.so: Routine minikin::LineBreaker::setText() at /b/build/slave/Linux_Engine/build/src/out/android_release/../../flutter/third_party/txt/src/minikin/LineBreaker.cpp:74 Stack frame #02 pc 002d95b5 /data/app/com.sankuai.meituan.takeoutnew-1/lib/arm/libflutter.so: Routine txt::Paragraph::ComputeLineBreaks() at /b/build/slave/Linux_Engine/build/src/out/android_release/../../flutter/third_party/txt/src/txt/paragraph.cc:273 Stack frame #03 pc 002d9f33 /data/app/com.sankuai.meituan.takeoutnew-1/lib/arm/libflutter.so: Routine txt::Paragraph::Layout(double, bool) at /b/build/slave/Linux_Engine/build/src/out/android_release/../../flutter/third_party/txt/src/txt/paragraph.cc:428 Stack frame #04 pc 00068e6d /data/app/com.sankuai.meituan.takeoutnew-1/lib/arm/libflutter.so: Routine blink::ParagraphImplTxt::layout(double) at /b/build/slave/Linux_Engine/build/src/out/android_release/../../flutter/lib/ui/text/paragraph_impl_txt.cc:54 Stack frame #05 pc 00067da5 /data/app/com.sankuai.meituan.takeoutnew-1/lib/arm/libflutter.so: Routine tonic::DartDispatcher<tonic::IndicesHolder<0u>, void (blink::Paragraph::*)(double)>::Dispatch(void (blink::Paragraph::*)(double)) at /b/build/slave/Linux_Engine/build/src/out/android_release/../../topaz/lib/tonic/dart_args.h:150 Stack frame #06 pc 00067d5f /data/app/com.sankuai.meituan.takeoutnew-1/lib/arm/libflutter.so: Routine void tonic::DartCall<void (blink::Paragraph::*)(double)>(void (blink::Paragraph::*)(double), _Dart_NativeArguments*) at /b/build/slave/Linux_Engine/build/src/out/android_release/../../topaz/lib/tonic/dart_args.h:198 Stack frame #07 pc 003b1877 /data/app/com.sankuai.meituan.takeoutnew-1/lib/arm/libflutter.so: Routine dart::NativeEntry::AutoScopeNativeCallWrapperNoStackCheck(_Dart_NativeArguments*, void (*)(_Dart_NativeArguments*)) at /b/build/slave/Linux_Engine/build/src/out/android_release/../../third_party/dart/runtime/vm/native_entry.cc:198 Stack frame #08 pc 003b1db5 /data/app/com.sankuai.meituan.takeoutnew-1/lib/arm/libflutter.so: Routine dart::NativeEntry::LinkNativeCall(_Dart_NativeArguments*) at /b/build/slave/Linux_Engine/build/src/out/android_release/../../third_party/dart/runtime/vm/native_entry.cc:348 Stack frame #09 pc 0000241c /data/data/com.sankuai.meituan.takeoutnew/app_flutter/vm_snapshot_instr 

Dart异常则比较简单,默认情况下Dart代码在编译成机器码时并没有去除符号表信息,所以Dart的异常堆栈本身就可以标识真实发生异常的代码文件和行数信息:

FlutterException: type '_InternalLinkedHashMap<dynamic, dynamic>' is not a subtype of type 'num' in type cast #0 _$CategoryGroupFromJson (package:wm_app/lib/all_category/model/category_model.g.dart:29) #1 new CategoryGroup.fromJson (package:wm_app/all_category/model/category_model.dart:51) #2 _$CategoryListDataFromJson.<anonymous closure> (package:wm_app/lib/all_category/model/category_model.g.dart:5) #3 MappedListIterable.elementAt (dart:_internal/iterable.dart:414) #4 ListIterable.toList (dart:_internal/iterable.dart:219) #5 _$CategoryListDataFromJson (package:wm_app/lib/all_category/model/category_model.g.dart:6) #6 new CategoryListData.fromJson (package:wm_app/all_category/model/category_model.dart:19) #7 _$AllCategoryResponseFromJson (package:wm_app/lib/all_category/model/category_model.g.dart:19) #8 new AllCategoryResponse.fromJson (package:wm_app/all_category/model/category_model.dart:29) #9 AllCategoryPage.build.<anonymous closure> (package:wm_app/all_category/category_page.dart:46) <asynchronous suspension> #10 _WaimaiLoadingState.build (package:wm_app/all_category/widgets/progressive_loading_page.dart:51) #11 StatefulElement.build (package:flutter/src/widgets/framework.dart:3730) #12 ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3642) #13 Element.rebuild (package:flutter/src/widgets/framework.dart:3495) #14 BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2242) #15 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&RendererBinding&WidgetsBinding.drawFrame (package:flutter/src/widgets/binding.dart:626) #16 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding&PaintingBinding&RendererBinding._handlePersistentFrameCallback (package:flutter/src/rendering/binding.dart:208) #17 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding._invokeFrameCallback (package:flutter/src/scheduler/binding.dart:990) #18 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding.handleDrawFrame (package:flutter/src/scheduler/binding.dart:930) #19 _WidgetsFlutterBinding&BindingBase&GestureBinding&ServicesBinding&SchedulerBinding._handleDrawFrame (package:flutter/src/scheduler/binding.dart:842) #20 _rootRun (dart:async/zone.dart:1126) #21 _CustomZone.run (dart:async/zone.dart:1023) #22 _CustomZone.runGuarded (dart:async/zone.dart:925) #23 _invoke (dart:ui/hooks.dart:122) #24 _drawFrame (dart:ui/hooks.dart:109) 

原文作者:美团技术团队
原文链接:https://zhuanlan.zhihu.com/p/41731950
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原文链接:https://yq.aliyun.com/articles/738995
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